In 1970, H. B. Bartl et al. reported a peculiar feature that among sixty-six oxygen atoms within a unit cell containing two molecules in C12A7 crystals, two oxygen atoms are clathrated as “free oxygen” in a space of each cage (cage structure) existing in the crystals (Non-Patent Document 1). Afterwards, it was found that the free oxygen can be replaced with various types of anions.
Hosono, who is one of the inventors of the present invention, has discovered that about 1×1019 cm−3 of O2− are clathrated by measuring the electron spin resonance of C12A7 crystals synthesized by a solid-phase reaction using CaCO3 and Al2O3 or Al(OH)3 as starting materials in air at a temperature of 1,200° C. Hosono has proposed a model in which some free oxygen ions exist in each cage in the form of O2− (Non-Patent Documents 2 and 3).
The present inventors have discovered that a C12A7 compound which clathrates active oxygen species in a high concentration of 1020 cm−3 or more can be produced by a solid-phase reaction using a starting material prepared by mixing calcium and aluminum in an atomic equivalence ratio of about 12:14 at a controlled temperature under a controlled atmosphere. A patent application related to inventions regarding the compound itself, a process for producing the compound, means for extracting clathrate ions, a method of identifying active oxygen ion radicals, and a use of the compound has been filed (Patent Document 1). Furthermore, the present inventors have filed a patent application related to inventions regarding C12A7 compound single crystal containing a high concentration of active oxygen species and a process for producing C12A7 single crystal free of air bubbles (Patent Document 2).
Furthermore, the present inventors have found a method of clathrating or extracting active oxygen at about 700° C. by controlling the concentration of anion other than oxygen ions, such as OH− ions in the compound and have filed a patent application related to this invention (Patent Document 3). In addition, the present inventors have synthesized a C12A7 compound containing OH− ions in a concentration of 1021 cm−3 or more by sintering in an oxygen atmosphere a C12A7 compound powder obtained by a hydration reaction in water, in a solvent containing moisture, or in a gas containing water vapor, and have filed a patent application regarding the inventions related to the compound itself, a process for producing the compound, a method of identifying OH− ions, and a use of the compound (Patent Document 4).
In addition, the present inventors have found that hydrogen anions can be extracted in vacuum by applying an electric field to a C12A7 compound containing hydrogen anions. Furthermore, the present inventors have discovered that green coloring of the compound occurs by irradiation of ultraviolet rays or X-rays, at the same time, the compound is permanently changed from an electrical insulator to an electrical conductor, and the compound can be returned to the insulating state again by heating or by irradiation of a strong visible light. The present inventors have filed a patent application regarding an invention related to the application of this finding (Patent Document 5 and Non-Patent Document 4). In addition, the present inventors have filed a patent application regarding inventions related to a method of permanently converting C12A7, which is originally an electrical insulator, to an electrical conductor by allowing an alkali metal to clathrate in C12A7, and a material that can exhibit an electrical conduction property in air at room temperature (Patent Document 6). In addition, an effect of hydrogen reduction on the electrical conduction property of C12A7 has been reported (Non-Patent Document 5).
Electride compounds are a concept which was first proposed by J. L. Dye (Non-Patent Document 6), and realized for the first time by forming, for example, a compound containing a crown ether as a cation and electrons as anions. It is known that electrides exhibit an electrical conduction property due to hopping of electrons contained as anions. Thereafter, some organic electride compounds were found, but all of these compounds are stable only at low temperatures of about 200K or lower and significantly unstable compounds which react with air or water.
Recently, an inorganic electride compound prepared by doping cesium into a zeolite compound powder having a silica skeleton was found. This silica zeolite plays a role of complexation as in a crown ether to achieve stabilization at room temperature. However, this compound is also highly reactive with moisture and chemically unstable (Non-Patent Document 7). Furthermore, a diode utilizing an excellent electron-releasing property of an electride compound has been proposed (Non-Patent Document 8). However, since electride compounds which have been obtained to date are thermally and chemically unstable, the proposed vacuum diode can be operated only at low temperatures.
The present inventors have found that a C12A7 compound having an electric conductivity of less than 103 S/cm can be obtained by heat-treating C12A7 single crystal in an alkali metal vapor and an alkaline-earth metal vapor at a high temperature, performing ion implantation of an inert ion into C12A7 single crystal, or solidifying a C12A7 compound directly from a melt thereof in a reducing atmosphere containing carbon, and have filed a patent application regarding a related invention (Patent Document 9). Almost all free oxygen ions in these C12A7 compounds having a high electric conductivity are replaced with electrons. Such C12A7 compounds are substantially represented by [Ca24Al28O64]4+(4e−), and can be considered to be inorganic electride compounds (Non-Patent Document 8). The present inventors have further filed a patent application regarding an invention related to a proton-electron mixed conductor obtained by heat-treating C12A7 or a compound similar thereto in a hydrogen atmosphere (Patent Document 9) and an invention related to an electroconductive complex oxide crystalline compound (Patent Document 10).
Patent Document 1: Japanese Patent Application No. 2001-49524 (Japanese Unexamined Patent Application Publication No. 2002-3218)
Patent Document 2: Japanese Patent Application No. 2001-226843 (Japanese Unexamined Patent Application Publication No. 2003-40697)
Patent Document 3: Japanese Patent Application No. 2001-321251 (Japanese Unexamined Patent Application Publication No. 2003-128415)
Patent Document 4: Japanese Patent Application No. 2001-117546 (Japanese Unexamined Patent Application Publication No. 2002-316867)
Patent Document 5: Japanese Patent Application No. 2003-586095 (Domestic re-publication of PCT International Publication for patent application Ser. No. 03/089,373)
Patent Document 6: Japanese Patent Application No. 2002-188561 (Japanese Unexamined Patent Application Publication No. 2004-26608)
Patent Document 7: U.S. Pat. No. 5,675,972, specification and drawings
Patent Document 8: PCT/JP2004/001507 (WO2005/000741 A1)
Patent Document 9: Japanese Patent Application No. 2003-209138 (Japanese Unexamined Patent Application Publication No. 2005-67915)
Patent Document 10: Japanese Patent Application No. 2004-136942 (Japanese Unexamined Patent Application Publication No. 2005-314196)
Non-Patent Document 1: H. B. Bartl and T. Scheller, Neuses Jarhrb. Mineral, Monatsh. 1970, 547
Non-Patent Document 2: H. Hosono and Y. Abe, Inorg. Chem. 26, 1193, 1987
Non-Patent Document 3: Hideo Hosono et al., “Zairyo Kagaku” (Materials science) Vol. 33, No. 4, p. 171-172, 1996
Non-Patent Document 4: K. Hayashi, Satoru Matsuishi, Toshio Kamiya, Masahiro Hirano, Hideo Hosono, Nature Vol. 419, No. 6906, pp. 462-465, 3 Oct. 2002
Non-Patent Document 5: A. R. West, et al., Solid State Ionics, Vol. 59, pages 257-262 (1993)
Non-Patent Document 6: J. Tehan, B. L. Barrett, J. L. Dye, J. Am. Chem. Soc., 96, 7203-7208 (1974)
Non-Patent Document 7: Ichimura, J. L. Dye, M. A. Camblor, L. A. Villaescusa, J. Am. Chem. Soc., 124, 1170, (2002)
Non-Patent Document 8: S. Matsuishi, Y. Toda, M. Miyakawa, K. Hayashi, T. Kamiya, M Hirano, I. Tanaka, and H. Hosono, Science 301626-629 (2003)